It is well known to provide a roll up, or retractable screen assembly that can extend across a window or door opening. In most cases, a flexible screen is wound about a wooden, metal or plastic rod or pole which is positioned in a substantially horizontal manner such that the screen assembly can be pulled up or pulled down. A Holland blind is an example of this type of screen assembly.
It is also known to provide a retractable screen assembly that can move sideways across a window or door cavity, and with this type of assembly, the screen (for instance a mesh) can be wound about a substantially vertical rod or pole (typically located at one side of the cavity).
There is a general requirement that the flexible screen is placed under tension to keep it relatively taut when pulled from the retracted position to the extended position. For vertically moving (that is up and down moving) screen assemblies, this can be achieved by providing some type of weight on the bottom edge rail of the screen. However, for horizontally moving screen assemblies, this cannot be easily done, as attachment of a rail to the lower edge of the screen will prevent the screen from being rolled up about the vertical rod.
It is not satisfactory to simply turn a vertically moving screen assembly on its side to make a horizontally (sideways) moving screen assembly. For instance, one disadvantage is that gravity cannot be used to tension the screen material (as is the case with vertically moving screens). Another disadvantage is that the screen material has a tendency to sag as it is pulled across which is unsightly and can create gaps and openings and damage to the mechanism.
A sideways moving screen which has a desirable configuration is one which has a flexible screen material (for example, an insect screen material) wound about a vertical rod and which can be pulled across a window or door opening. Because the screen material is wound about the rod, it cannot be attached to upper and lower horizontal frame members. Instead the screen material is attached at one vertical edge to the rod (the rod can be called the “second edge member”), and at the other vertical edge to a stile (the stile can be called the “first edge member”). The top and bottom edges are free.
A disadvantage with the construction of a large piece of screen material (e.g. fabric) supported on only two edges (that is with the other two edges largely unsupported) is that the fabric between the edge members is subject to the effects of gravity and will sag under its own weight. Many fabrics are made from individual strands of the base material woven together to make one coherent piece. Insect screening is one such example as are various types of shade screen materials.
The screen material has substantially horizontally orientated longitudinal fibres which extend from one edge member to the other edge member.
It is extremely desirable that the screen material (and particularly the longitudinal fibres) does not exhibit any sagging especially when extended across large window or door openings. The screen material should maintain an attractive “flat” appearance.
This is usually achieved by providing a tensioning means (typically a spring or weight associated with the rod) to tension the screen material to keep is taut and flat.
One desirable object of the present invention is aimed at means of keeping a flat appearance in a piece of fabric that is supported by edge members on only two opposing edges, these edge members being a substantial distance apart—typically 1.5 meters or more.
With vertically operating screens (up and down movement), the lower horizontal edge can be attached to a weighted bottom rail to provide relatively even tension in the fabric and thus achieve a flat appearance. However with sideways (horizontally) moving screens, upper and lower rails (frame members) are not possible and the screen material has a tendency to sag, and to reduce the sagging, various attempts have been made to provide tension in the screen material.
A known solution to provide tension with horizontally moving screen assemblies is to provide some form of spring to maintain tension in the screen material. The spring can be placed within a hollow tube about which the screen material is wound. Extension (unrolling) of the screen material from the tube causes the spring to be increasingly “wound up” to create tension (a “pullback” force) in the screen material.
A disadvantage with this arrangement is that the amount of tension increases as more of the screen material is unwound from the tube. This means that it can become progressively more difficult to pull the screen material further across the window or door cavity from one side to the other. It should be appreciated that one use of the screen assembly is across rather large door cavities that can have a length of between 3-6 m, and usually contain bifold doors, multi-sliding doors or French doors that have a length of 1.2-2 m. Thus, it can become very difficult to pull a screen across the door cavity as the tension can become too large—especially with a single screen spanning 2 m or more. If the tension is reduced to compensate, the screen material can sag due to insufficient tension, or have sag lines.
Another disadvantage with the above type of arrangement is that the increasing tension can cause damage (for instance premature stretching) of the screen material, especially if the screen material is relatively flimsy. There is an advantage in having screen material which is relatively thin (and therefore may be somewhat flimsy), as it allows a greater length of material to be wound about the tube without making the diameter to large to be neatly hidden away in one side of the cavity.
An attempted solution to this problem has been to introduce some form of brake. However, any form of brake can increase the number of parts in the assembly, and require constant maintenance and possible replacement, can fail under wet conditions or if debris or grime come into contact with the brake, and therefore the concept of having some form of brake is generally undesirable
It is also known to try to balance the tension on the screen, in essence, to try to reduce the increasing tension force as the screen is pulled across the window or door cavity. Various arrangements of counterweights have been tried to provide some type of balance to the tension force. These counterweights may comprise a weighted rod attached to the end of a line member. Other arrangements use various types of “counter” spring arrangements to provide a balance.
A disadvantage with the use of counterweights is the problem with inertia. To explain, when the screen is in the extended position, and typically pulled across the window or door cavity and latched to the other side, if it is desired to open the screen by pulling the screen partially back away from the other side of the window and door cavity, doing so will cause acceleration or deceleration of the counterweights, and the consequence of this is that the screen will always feel “heavy” to operate quickly which is quite undesirable.
A disadvantage with the use of a “counter” spring is that the spring can only produce a complete neutralisation or balance of the tension at only one extended position. Put differently, with the use of a counter spring, the screen can be pulled across and pulled back more easily than without a counter spring, but if the screen is let go, the “balance” position will be somewhere across the door or window cavity. While this arrangement has some advantages, there is still the general disadvantage that there will always be some resistance to movement of the screen across the window or door cavity except at the one particular “balance” point. It is generally not possible to vary the counter spring in a continuous manner such that the screen is always balanced no matter where the screen is across the door cavity.
It is also known to provide a screen that can be pulled across a door or window or other type of cavity and where there is also provided a line member and pulleys etc to assist in the extension and the retraction of the screen. For a horizontally extending screen, it is known to have a vertical rod, typically at one end of the cavity, and about which to screen material can be wound/unwound, and pulleys or similar devices at the other end of the cavity, with a line member (typically a steel wire, plastic wire etc) connecting the various parts.
As mentioned previously, there are many instances where it is desirable to have a rather large flexible screen (for instance an insect screen) which can cover a rather large area but where there is an advantage in being able to retract the screen. This type of assembly is useful for a “flexible” screen door. The door can be slightly opened to allow passage, and opening and closing of the flexible screen door results in the screen material being wound onto and off a roll. This type of arrangement is known. It is found that the flexible mesh (e.g. insect screen) is susceptible to a degree of sag at least in the top of the screen. It is found that the sag becomes progressively more pronounced as the screen is made larger. For some small screen doors, the sag, while being present, is not very pronounced, but for larger screen doors (and of course other types of flexible membranes or members), the sagging can become pronounced.
In respect of high-quality and expensive screen arrangements, any sag or “soft spots” in the mesh is highly undesirable and must be controlled or eliminated if at all possible.
It is found that simply increasing the tension in the flexible material is not a total solution as it can cause damage to the material, and it is found that for larger screen areas, even a high degree of tension does not overcome sag zones or soft spots in the mesh.
One key element in achieving a flat appearance in a wide expanse of fabric supported on only two opposing edges is to ensure that all fibres that span between the two supporting edges are essentially in one plane and parallel. A fibre with little or no inherent stiffness suspended between two points will hang in a particular shape known as a Catenary (see FIG. 7). If inherent stiffness is ignored, the exact shape of the hanging fibre is dependent upon its own weight (acting under gravity) and the tension in the fibre.
Because the fibres in a piece of fabric in a vertical orientation all tend to hang in such a Catenary shape between the vertical edge supports, to ensure a flat appearance it is important to get the tension in the fibres to be as close to uniform as possible so that the fibres will hang parallel and in the one plane. If for example the bottom fibres were under higher tension and the fibres above them were under lesser tension, the upper fibres would deflect less than the fibres at the bottom and would tend to sit on top of the lower fibres. This phenomenon can result in the upper fibres tending to fall to either side of the lower fibres such that they do not lie in the same plane and this results in the piece of fabric no longer being or looking flat. This can result in soft or sagging spots in the fabric which looks unsightly when the fabric is unfurled (unrolled) and may also result in wrinkling of the cloth when it is rolled up on the rod member.
In the case of a screen product, it is advantageous to have some tension in the flexible membrane so that it does not sag too much. More tension equals less sag, but there are limitations to just how much tension can be applied to the mesh—too much and the strands will break. There are also practical limitations to how to apply substantial tension across a wide span of membrane.
Practical limitations on tension result in a flexible membrane that may sag 25 mm or more across an aperture of 2.5 meters in width. This is a lot of distortion to accommodate, and in most screening products it is important to ensure that the top and bottom edges of the membrane remain covered by an edging strip or inserted into a channel—this will ensure light block out or prevent insects from traveling around the extremities of the membrane. To have an edging strip or channel deep enough to accommodate 25 mm of sag plus a good edge coverage for a safety margin is not very practical, and a method of ensuring a substantially straight edge at the top and bottom of the screen as it rolls in or out and also when it is fully deployed is important.
It is envisaged that the screen material will mostly comprise a woven material which will have longitudinal fibres extending from one side edge of the screen material to the other side edge of the screen material. An insect screen is an example of a woven material. However, it should be appreciated that the present invention is applicable to other screen materials of the type that stretch (even to a small degree) and therefore have dead spots or sag spots in the screen material. These materials may comprise non-woven screens such as plastic sheets, some types of metallic films and the like.
Therefore, there would be an advantage if it were possible to devise a system that would allow larger (but also including smaller) flexible membranes (e.g. insect screens) to be suitably taut or tensioned over the entire area to eliminate or at least reduce the existence of sag or soft spots.
It will be clearly understood that, if a prior art publication is referred to herein, this reference does not constitute an admission that the publication forms part of the common general knowledge in the art in Australia or in any other country.